The present invention relates to horizontal burning metal halide lamps with improved efficiency and improved color qualities.
It is well known that a dramatic difference exists in the color qualities of the light produced as a function of the vertical or horizontal positioning of metal halide arc lamps used for general lighting applications When such lamps are mounted in the preferable horizontal position, the arc tends to bow upwardly, tending to overheat the upper part of the arc tube and to create a cold spot along the bottom of the arc tube. The overheating tends to reduce the life of the arc tube and to degrade both the intensity and quality of color over the life of the lamp.
Metal halides tend to condense in the cold spots because of the temperature differential between the upper and lower regions of the arc tube. This condensation decreases the vapor pressure of the various elements of the lamp fill material which determine the color of the lamp, resulting in degradation of intensity and color quality.
The generally known prior art has attempted to accommodate the problem of a bow in the arc in several ways. One such approach has been the use of an electromagnetic field to hold the arc in a horizontal position. This approach generally results in a complex structure and has not found favor because of increased size and expense.
In another prior art approach such as disclosed in the Koury U.S. Pat. No. 3,858,078 dated Dec. 31, 1974 and the Karlotski, et al., U.S. Pat. No. 4,498,027 dated Feb. 5, 1985, the arc tube is arched to effectively center the arc tube about the bowed arc. However, the arch in the tube increases the effective diameter of the arc tube and thus the lamp, making the lamp unsuitable for many compact fixtures. The use of an arched tube also creates cold spots at both ends in the regions behind the electrodes, and the necessity for arching the tube increases the complexity and cost of manufacturing the lamp.
Still other known approaches, as disclosed for example in the Rigden U.S. Pat. No. 4,232,243 dated Nov. 4, 1980, and Howles, et al., U.S. Pat. No. 4,001,623 dated Jan. 4, 1977, lower the elevation of the electrodes within the arc tube to provide additional room for the upward bowing of the arc in the operation of the lamp. While lowering the electrodes reduces the overheating of the arc tube at its upper extremity and raises the temperature in the lower region, it generally does not raise the temperature of the lower regions of the arc tube sufficiently to obviate the problem of lamp fill condensation and associated reduction in vapor pressures particularly at the cold spots below and behind the electrodes at both ends of the arc tube.
In addition, the placement of the electrodes within the lower portion of the arc tube of the traditional symmetrical pinch design is difficult to control in manufacturing, and any tilting of the arc tube from a position other than horizontal creates wide variances in the color temperature of the discharge. Further, some of the metal halides typically condense at the cold spots in the seam of the pinch below the electrodes creating problems with the seam. Thus, lamp designs with lowered electrodes have not achieved a great deal of commercial success.
In attacking the cold spot problem, it is a common practice in metal halide arc discharge lamps to apply heat reflective coatings to the ends of the arc tube to reduce condensation of lamp fill material behind the electrodes. As disclosed for example in the Johnson U.S. Pat. No. 4,281,267 dated July 28, 1981, this heat reflective coating on the ends of an arched arc tube may be reduced in the area behind the electrodes in the upper surface of the tube to reduce the internal reflections and increase the upward exposure of a greater portion of the arc.
As disclosed in the Ramberg U.S. Pat. No. 3,963,951 dated June 15, 1976, a longitudinal stripe of heat reflective material also may be provided along the bottom of the arc tube to reduce condensation of the metal halides in the center thereof at the farthest distance from the upward bow in the arc. The use of such coatings is helpful in the elimination of the cold spots but does not impact on the hot spot problem. As earlier explained, the arched tube may require a larger diameter envelope and a corresponding larger fixture.
In another prior art approach to the cold spot problem, and in lieu of a coating, the arc tube may be constructed out of an ultraviolet absorbing quartz glass over the lower portion thereof. Such construction is disclosed, e.g., in the Meulemans, et el., U.S. Pat. No. 4,307,315 dated Dec. 22, 1981. While the temperature of the lower portion of the lamp is increased, this approach does not obviate the hot spot at the top of the lamp. In addition, multiple glass arc tubes are difficult to manufacture and relatively expensive.
It is, accordingly, an object of the present invention to obviate these and other problems of the known prior art horizontal burning metal halide lamps, and to provide a novel metal halide lamp for general lighting applications.
Another object of the present invention is to provide a novel horizontal burning metal halide lamp with greater efficacy and improved color uniformity.
Yet another object of the present invention is to provide a novel metal halide lamp less sensitive to changes in the position of the lamp from the horizontal.
A further object of the present invention is to provide a novel horizontal burning metal halide lamp with the electrodes lowered within the arc tube.
A still further object of the present invention is to provide an arc tube for a horizontal burning lamp with novel shapes and dimensions of the end chamber.
Yet a further object of the present invention is to provide a novel arc tube for a horizontal burning lamp with the cold spot located along the lower wall of the arc tube between the electrodes.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the metal halide lamp art upon a perusal of the claims and the following detailed description when read in conjunction with the appended drawings.